It's a cetane booster, which I'm guessing is like an octane booster in petrol.
I've been using it for 2-3 months now in my 2.0TDI, can't say I really notice any effect when running BP Ultimate diesel, but it does seem to perk up Tesco diesel very nicely.
For info to BP ultimate users, Total is bringing out a new diesel called Excellium which is much along the lines of BP Ultimate, the unleaded petrol version is already available in petrol stations (seems to be southern england only at present) but I've yet to find the diesel version.